The pulse tube refrigerator is a cryocooler, similar to Stirling and Gifford-McMahon refrigerators, which derive cooling from the compression and expansion of gas. However, unlike the Stirling and Gifford-McMahon (G-M) systems, in which the gas expansion work is transferred out of the expansion space by a solid expansion piston or displacer, pulse tube refrigerators have no moving parts in their cold end, but rather an oscillating gas column within the pulse tube (called a gas piston) that functions as a compressible displacer. The elimination of moving parts in the cold end of pulse tube refrigerators allows a significant reduction of vibration, as well as greater reliability and lifetime, and is thus potentially very useful in many applications, both military and commercial.
Most military type applications use Stirling pulse tubes that operate at frequencies of 20 to 60 Hz, and as a result of the high speed are small, but are limited to temperatures above about 20 K. Cryogenic temperatures as cold as 3 K are achievable using two stage GM type pulse tube refrigerators which typically operate at 1 to 2 Hz. Cryocoolers operating at either 10 K or 4 K are presently used to cool the superconducting magnets used in magnetic resonance imaging (MRI) systems. A minimum temperature of about 12 K is highly desirable in such commercial applications as cryopumps, which are often used to purge gases from semiconductor fabrication vacuum chambers.
In each of these applications, there exist continued efforts to further reduce the level of vibration produced by the cryogenic refrigerator. Pulse tube refrigeration systems that are characterized by lower vibration will greatly increase the reliability and lifetime of cryocoolers.
Most vacuum chamber processes are very sensitive to vibration. With processes requiring accuracy to within nanometers, any motion can result in production defects. Conventional vacuum chamber pumps involve a system of moving parts that can cause the movement of production elements. What is needed is a way to reduce the vibration generated by a cryopump.
Cryopumps must be cooled to temperatures as low as 12 K to condense and solidify or adsorb various species of chamber gases onto one or more cryopanels. Conventional refrigerators used for obtaining these low temperatures are Gifford McMahon, GM, cycle systems, however, these systems have significantly more vibration than a pulse tube. What is needed is a way to cool a cryopump using a two-stage pulse tube.
Two-stage pulse tube refrigerators need to have the hot ends of the pulse tubes at the top in order to avoid convective losses within the pulse tubes. It is also most common to have a bulky valve mechanism on top of the cooler so the necessary valves can be integrated into a common housing and the heat that is generated at the hot ends of the pulse tubes can be transferred to the low pressure gas returning to the compressor within this same housing. In most cryopump applications it is preferred to mount the cryopump below the vacuum chamber, with a minimum space between the cryopump housing and the vacuum chamber.
A conventional two-stage pulse tube refrigerator with double orifice phase control requires a buffer volume that is relatively large. It is possible to have a significantly smaller buffer volume that can be integrated with the hot ends of the pulse tubes by using inter-phase control in combination with double orifice phase control. The buffer volume compensates for the difference in the volumes between the two stages of the pulse tubes.
Gao et al., U.S. Pat. No. 5,974,807, dated Nov. 2, 1999 and entitled “Pulse Tube Refrigerator” describes a pulse tube refrigerator capable of generating cryogenic temperatures of below 10 K that includes first and second refrigeration stages. Each stage includes a pulse tube and an associated regenerator provided at the low temperature side of the pulse tube. A pressure fluctuation generator having a compressor and a first to a fourth valve is provided at the high temperature side of each regenerator. The high temperature ends of each pulse tube are connected by a continuous channel, while the high temperature ends of each pulse tube and the high temperature ends of each regenerator are connected by a by-pass channel. A magnetic material having a rare-earth element and a transition metal is used as a regenerative material for the regenerator.
When pressure fluctuation is generated in each pulse tube at the phase difference angle of 180 degrees, respectively, a working gas is transferred between the high temperature ends of each pulse tube as controlled by an active valve, and between the high temperature ends of each pulse tube and its associated regenerator as controlled by a passive valve. This optimizes the phase angle between the pressure fluctuation in each pulse tube and the displacement of the working gas.
Li, U.S. Pat. No. 5,927,081, dated Jul. 27, 1999 and entitled “Pulse Tube Refrigerator and its Running Method” describes a method of running a pulse tube refrigerator that has a regenerator and a pulse tube each defining a high temperature end and a low temperature end, the low temperature ends of the regenerator and the pulse tube communicating with each other, and the high temperature end of the regenerator being connected to a gas compressor. A cold area is formed at the low temperature ends by periodically supplying working gas from the high temperature end of the regenerator to the regenerator and recovering the working gas from the regenerator. The temperature of the low temperature ends is raised by steadily, pulsatively, or intermittently flowing gas in one direction through a communicating area between the low temperature ends of the regenerator and the pulse tube.
Matsui et al., U.S. Pat. No. 5,845,498, Dated Dec. 8, 1998 and entitled “Pulse Tube Refrigerator” describes a pulse tube refrigerator where the cryostat includes regenerators and pulse tubes. Each regenerator has a cold stage at an upper end thereof. Each pulse tube has a low-temperature end portion at a lower end thereof and a high-temperature end portion thereof, the low-temperature end portion being located lower than the cold stage. The cold stage and the low-temperature end portion are connected to each other through a line whose cubic volume is substantially negligible in comparison with that of the pulse tube. Since the pulse tube has working gas of relatively high (should be low) density in an upper portion thereof and working gas of relatively low (should be high) density in a lower portion thereof, there is no convection of working gas induced by the gravity.
Chan, C. K. and Tward, E., U.S. Pat. No. 5,107,683, dated Apr. 28, 1992 and entitled “Multistage Pulse Tube Cooler” describes a multistage pulse tube cooler in which a portion of the heat from each successively lower-temperature pulse tube cooler is rejected to a heat sink other than the preceding higher-temperature pulse tube cooler, thus substantially improving the overall efficiency of the multistage cooler. Multistage pulse tube coolers of the prior art reject all the heat from each successively lower-temperature pulse tube cooler to the preceding higher-temperature pulse tube cooler, thus imposing a large cooling load on the higher-temperature pulse tube coolers which considerably reduces the overall efficiency of the cooler.
Zhu, S. and Wu, P., “Double inlet pulse tube refrigerators: an important improvement”, Cryogenics, vol. 30 (1990), p. 514 describe the second orifice and how it improves the performance of a single stage pulse tube. A. Watanabe, G. W. Swift, and J. G. Brisson, Superfluid orifice pulse tube below 1 Kelvin, Advances in Cryogenic Engineering, Vol. 41B, pp. 1519–1526 (1996) describe inter-phase control. It discusses a very low temperature Stirling cycle cooler that has one passive orifice between two identical pulse tubes. J. L. Gao and Y. Matsubara, An inter-phasing pulse tube refrigerator for high refrigeration efficiency, in: “Proceedings of the 16th International Cryogenic Engineering Conference”, T. Haruyama, T. Mitsui and K. Yamafriji, ed., Eisevier Science, Oxford (1997), pp. 295–298 discuss identical dual 1, 2, and 3 stage pulse tubes with single active interconnect valves. C. K. Chan, C. B. Jaco, J. Raab, E. Tward, and M. Waterman, Miniature pulse tube cooler, Proc. 7th Int'l Cryocooler Conf., Air Force Report PL-CP-93-1001 (1993) pp. 113–124 describe a Stirling single stage pulse tube that is inline, thus the hot end of the pulse tube is remote from the regenerator inlet. It has double orifice control. Heat from the hot end of the pulse tube and buffer are rejected to the base at the regenerator inlet by conduction through the buffer housing which extends the full length of the pulse tube. The hot end of the pulse tube is not attached to the vacuum housing so the entire pulse tube assembly can be easily removed.
There continues to exist the need for a pulse tube cooled cryopump, where the refrigeration unit has an inline configuration with the hot ends of the pulse tubes on top and where there is easy access to the components of the refrigeration unit. It would be desirable to configure a two-stage pulse tube refrigerator so that the valve mechanism is below the cryopump housing, the regenerators and pulse tubes are inline with the hot ends of the pulse tubes on top, and there is a means to remove the heat that is generated at the hot ends of the pulse tubes. It is also desirable to have access to the components of the two-stage pulse tube refrigerators to permit the cryopanels and the pulse tubes to be removed.
It is an object of the present invention to provide a way to cool cryopanels in a cryopump using a two-stage pulse tube.
It is an object of the present invention to provide a design for an inline two-stage pulse tube refrigerator.
It is an object of the present invention to provide a way to remotely remove heat from the hot end of an inline two-stage pulse tube refrigerator.
It is an object of the present invention to provide a means to minimize the size of an inline two-stage pulse tube refrigerator.
It is an object of the present invention to allow easy servicing of the system by removably attaching the cryopanels to the pulse tube and the pulse tube itself from the cryopump housing.
It is an object of the present invention to reduce the vibration generated by a cryopump.
It is an object of the present invention to provide a long maintenance cycle before regular maintenance is required.
It is an object of the present invention to offer improved reliability relative to existing GM refrigerators.